You can update ID number using the "Upload users" functionality. The CSV file would have "username" and "idnumber" headings, so for example you would have:
username
idnumber
learner
learner2
if you wanted to update the ID number of user "learner" to "learner2".
This does require that the username you set up isn't changing (because that can't be bulk updated as far as I'm aware).